Ships equipped with pressurized cargo tanks supported on continuous shells

ABSTRACT

A continuous, suitably stiffened skirt is attached by its bottom edge to the top of the double bottom of a ship and beneath the shell or shells which constitute the support for each cargo tank. The top edge of the skirt, which has the same circumference as the bottom edge of the internal one of the shells which form the cargo tank support and is opposite the same, is attached to a suitably stiffened platform, preferably horizontal, which extends between top edge and the inner shell of the ship&#39;&#39;s hull. Between the skirt and platform, there are a number of flat plates perpendicular to the base plane of the ship and located opposite the double bottom floors and longitudinal girders. These flat plates are welded at their top and inside edges to the platform and skirt, respectively, and at their bottom edge to the top of the double bottom, while at the external edges, those opposite the double bottom floors are welded to the inner shell of the ship&#39;&#39;s hull, whereas the plates opposite the double bottom longitudinal girders are welded to the skirt of the adjacent tank or to the terminal transverse bulkheads of the cargo tank zone.

United States Patent 1 Menendez SHIPS EQUIPPED WITH PRESSURIZED CARGO TANKS SUPPORTED ON CONTINUOUS SHELLS Jaime Torroja Menendez, Las Arenas, Spain [75] Inventor:

[73] Assignee: Sener, Tecnica Industrial y Naval,

S.A., Las Arenas, Spain [22] Filed: Sept. 4, 1973 [21] Appl. No.: 393,896

[451 Aug. 19, 1975 Primary litaminer-Trygve M. Blix Assistant ExaminerCharles E. Frankfort Attorney, Agent, or FirmFleit & Jacobson 5 7] ABSTRACT A continuous, suitably stiffened skirt is attached by its bottom edge to the top of the double bottom of a ship and beneath the shell or shells which constitutethe support for each cargo tank. The top edge of the skirt, which has the same circumference as the bottom edge of the internal one of the shells which form the cargo tank support and is opposite the same, is attached to a suitably stiffened platform, preferably horizontal, which extends between top edge and the inner shell of the ships hull. Between the skirt and platform, there are a number of flat plates perpendicular to the base plane of the ship and located Opposite the double bottom floors and longitudinal girders. These flat plates are welded at their top and inside edges to the platform and skirt, respectively, and at their bottom edge to the top of the double bottom, while at the external edges, those opposite the double bottom floors are welded to the inner shell Of the ship's hull, whereas the plates opposite the double bottom longitudinal girders are welded to the skirt of the adjacent tank or to the terminal transverse bulkheads of the cargo tank zone.

21 Claims, 12 Drawing Figures PATENTED Aum 9191s SHEET 1 BF 5 PATENTED AUB1 9197s SZIZCT 2 5 TENTH] AUG 1 91975 Fig. 9

PATENTEB AUG-1 9 I975 SIIIZT 5 BF 5 SHIPS EQUIPPED WITH PRESSURIZED CARGO TANKS SUPPORTED ON CONTINUOUS SHELLS BACKGROUND OF THE INVENTION The present invention relates to improvements in ships equipped with pressurized independent cargo tanks, designed especially for the transportation of liquefied gases.

The transportation by sea of liquefied gases has raised a number of technical problems stemming chiefly from the movements of the ship and also from the low temperatures at which the material has to be transported in the majority of cases. This is the case, for example, with liquefied natural gas (LNG) which has to be transported at a pressure close to atmospheric and a temperature of around l60C.

All this has meant that the responsible authorities have insisted that the transportation of LNG should be effected in double-walled vessels so that in the event of rupture of the primary barrier the low-temperature cargo will be temporarily retained by an insulated secondary barrier thus preventing the temperature of the hull from reaching such low levels that there is a serious risk of brittle fracture. However, the construction of double-walled vessels makes the overall cost of the ship substantially more expensive so that the cost of transportation of the liquefied gas increases correspondingly.

Recently, various authorities and bodies concerned with the setting of standards have decided to accept the elimination of the secondary barrier subject to the proviso of certain security conditions. This change of attitude has opened up broad possibilities for reducing the cost of tanker vessels for the transportation of LNG, in which the secondary barrier, due to the enormous amount of labor it involves and the construction as well, makes up a considerable percentage of the total cost of the ship, and major efforts have been directed towards the construction of liquid gas transportation vessels not possessing a secondary barrier.

The most logical method of constructing such vessels while still complying with the requisite security regulations, is to design them as pressure vessels, in the form of unstiffened solids-of-revolution, perhaps the most suitable shape for which would be spherical. Tanks of this kind, in the form of solids-of-revolution, and more specifically spherical ones, nevertheless present major problems not merely of construction but also in terms of the system of support in the ship, due to the large size of the tanks.

Recently, two basic designs have been put forward within the context of pressure vessels having only a single barrier. Both designs employ spherical tanks, the chief difference between the two designs being the system of support. In accordance with one of the designs which have been developed, the forces acting upon each vessel are absorbed by a certain number of separate supports arranged along three great circles. Each individual support comprises a lever mechanism the purpose of which is to reduce to the minimum the bending forces transmitted by each support to the tank envelop.

The other design is basically a maritime version of the well tried approach according to which the tank is supported upon a continuous envelope which is a Solid of-revolution. Its chief characteristic is the special design of the equatorial portion which links the supporting frame to the top and bottom hemispheres of the tank. Following this second system, the present assignee has developed a type of support or bracket, which is described in Spanish Patent No. 40l ,734, which comprises a peripheral structure attached to the tank and to a certain extent forming part of the wall thereof, and two continuous skirts or shells which are unreinforced, attached by one of their edges to said peripheral structure and by the other to the ships structure.

SUMMARY OF THE INVENTION system suitable for the assembly of large-size tanks for the largest ships envisaged.

In accordance with the invention, on the top of the double bottom of the ship, and beneath the shell or shells which constitute the support for each tank, there is arranged a continuous, suitably stiffened skirt attached by its bottom edge of the top of the double bottom of the ship, and at its top edge, which has the same circumference as the bottom edge of the internal one of the shells which form the tank support and is opposite the same, there is attached a suitably stiffened platform, preferably horizontal, which extends between the top edge and the inner shell of the hull. Moreover, between the skirt and platforms there are a number of flat plates perpendicular to the base plane of the ship and duly stiffened, located opposite the double bottom floors and longitudinal girders. These flat plates are welded at their top and inside edges to the platform and skirt, respectively, and at their bottom edge to the top of the double bottom, while at the external edges, those opposite the double bottom floors are welded to the inner shell of the ships hull, whereas the plates opposite the double bottom longitudinal girders are welded to the skirt of the adjacent tank or to the terminal transverse bulkheads of the cargo tank zone.

With this disposition, the vertical forces are transmitted from the tank support to the skirt as normal forces and to the flat plates as shear forces along the whole length of the weld seams. From these the loads are then distributed to the floors, webframes, longitudinal girders, transverse bulkheads and the other structural elements of the ship. The horizontal dynamic forces are transmitted from the tank support to the ships structure principally as shear forces which are distributed over the whole of the horizontal platform. With this disposition, the ships structure retains its simple constitution and at the same time no stress concentrations are induced into the tank support or the ships structure.

On the inside of the aforementioned skirt there are a series of brackets fitted with faceplates and intermediate stiffeners. These brackets are coplanar with the aforementioned plates. The brackets are fixed to the skirt by the outside edge and by the lower edge to the top of the double bottom.

If it is desired to reinforce the joint, a small skirt may be put under the platform and opposite the external shell of the support, with a smaller height than that of the skirt and joined by its upper edge to the platform. This small skirt is also joined to the abovementioned plates on the lines of intersection of the plates and skirt by means of intermediate parts. Preferably the internal edge of the platform will be slightly inside the upper edge of the skirt.

In accordance with the invention and in order to get a better transmission of forces, an intermediate deck can be put between the ships inner and outer shells, facing the platform. The disposition of the platform can permit the suppression of the double bottom'of the ship in the zone outside the skirt, away from the proximity of the lower edge of the skirt. The lower edges of the said flat plates are then joined to the faceplates of the floors and longitudinal girders. For the same reason one can dispense with the inner shell of the ship, at least in the zones situated below the platform, the external edges of the said flat plates facing the floors being then joined to the face plates of the transverse web frames.

The skirt can have either a conical or cylindrical shape. The cylindrical shape is the better so long as the shape of the ship permits its use.

With the described construction, the double bottom zoneresembles, under each tank, a flat grillage that has a rectangular shape if there are brackets on the inside of the skirt, and a circular one if not. This grillage is supported throughout its entire perimeter and is subjected to hydraulic pressure which acts upon its lower face. The flat plates between the skirt and the platform transmit the forces to the transverse webframes as shear forces and to the floors and longitudinal girders as compression forces. The brackets on the inside of the skirt improve the distribution of the loads and reduce the spans of the double bottom panels.

The horizontal forces which act on the tanks, be they transverse or longitudinal, are transmitted through the support as shear stresses. At each level they are distributed following a cosine function, the maximum being in the plane perpendicular to the direction of the forces and the minimum (zero) beingin the plane of application of the forces. These shear stresses are carried through the platform to the skirt and from the skirt to the top of the double bottom.

For the maritime transportation of liquid products at low temperatures in pressurized tanks with relatively thin walls, the competent authorities demand the installation of a partial secondary barrier or drip tray used to catch any small leaks that may be produced as a result of the appearance of small cracks over a period of some 2 weeks so that the structure of the ship does not find itself subjected to excessively low temperatures.

The measures which have been commonly used up to now consist of covering the ships structure, in the zone below the tank, with a water-tight coat of insulating material able to withstand the low temperature of the collected liquid. This system presents the inconvenience that its construction must be performed on board, and the above work cannot be carried out at the same time as the other, i.e., the construction of the hull itself, and the mounting of the tanks and their supports.

In accordance with the invention and foreseeing the possibility of said leaks, a drip tray is placed beneath the tank which is independent of the ships structure and of the tank support. This system allows the mounting of the tank and its support and corresponding tray in a single operation. The assembly can be suspended, in which case the tray is fixed to the support, at least during the mounting stage, or alternatively the assembly can be floated by flooding the dock where the ship is being built, in which case the upper edge of the tray must stay above the flotation level of the assembly.

The mounting of the tanks by flotation with their corresponding support and tray can be done without a great deal of water in the construction dock, before assembling the part of the platform and skirts situated between each two consecutive tanks in order that through this gap the tanks can float to their respective locations. Once the tank is in position the platform and small skirts can be completed and then the next tank can be mounted.

So that the tray can be constructed without stiffeners and be kept without danger of buckling during the operation of floating the assembly, at least the surface of the central part of the said tray should be parallel to the lower external surface of the tank, so that the support between tank and tray is achieved through the corresponding insulation over an area of sufficient magnitude. The rest of the tray may have a conical shape, tangential to the surface of the central zone.

The tray will be provided with insulation either below or above it. In the first case, the tray will be constructed from a metallic material, capable of resisting the low temperature of the dripping liquid, preferably the same material as the tank. The insulating layer should be protected during the flotation by a water-proof film, for example, a plastic-based film.

In the second case, the tray may be constructed from a material which does not resist low temperatures, preferably' normal steel, since the insulating layer will protect it from the aforementioned low temperatures. However in this case the insulation layer must have a tight covering able to withstand the low temperatures of the dripping liquid. The tight covering of the insulation layer, when placed on the upper face of the tray, can be constituted of one or two superimposed layers of tight material.

In the first case, the layer of tight material can be bonded to the insulating layer covering it completely, or can be independent of it, and be attached in sealed fashion along the whole of its free edge to the insulating layer or to the drip tray and in this way achieve a tight assembly defined below by the tray and above by the tight layer. Also this tight layer can be joined to the insulating layer at its centre; This system allows the detection of leaks and cracks at any time in the tight layer and in the tray by means of introducing gas under pressure without having to penetrate the limited space between the tray and the tank to make a visual inspection.

If the tight covering is formed by two superimposed layers then they will be separate from the insulating layer and be joined at their free edges, forming a bag whose tightness can easily be checked by introducing gas under pressure. The bag thus formed will be attached at its contour to the tray and rest on the insulating layer. Also, in this case the two layers can be joined together, at a series of intermediate points, thus reducing the volume of gas needed to test the water tightness, and to the insulating layer.

In any case, the deformation of the layer or layers, which might be caused by the pressurized gas will be confined to the defined space between the tank and the tray. The layer or layers that form the tight covering must be of a material that will withstand the low temperatures of any escaping liquid, for example, Mylar, reinforced or not with glass-fiber cloth.

No stresses will be caused in the tight covering by the low temperature of any escaping liquid, whether it is composed of one layer or two, because any shrinkage can be absorbed by the deformation of those parts of the layer or layers which reproduce the shape of the knuckle between the edge of the tray and the surrounding retention brim of the latter.

In the case where the insulating layer is on the upper face of the tray, said layer may be continuous or made of performed parts or panels. In either case, the layer can be formed of an expanded polyurethane, polyvinyl chloride, polystyrene, or any other suitable material that meets the thermal requirements and presents a certain resistance to compression.

If the insulating layer is formed by prefabricated pieces or panels, these will have a reduced surface area and so shrinkage caused by low temperatures, away from the upper face, will not produce grooves of excessive width which might cause bending stresses in the layer on layers of the tight covering when these layers have to withstand the liquid pressure coming from a possible leak in the tank.

With the same purpose, if the insulating layer is continuous, grooves will be machined in it in at least two different directions. These grooves should not penetrate the whole thickness of the material and should define intermediate portions that can contract under the low temperatures without giving rise to the formation of excessively wide intermediate grooves which would result in bending stresses in the layer or layers of the tight covering caused by the pressure of the escaping liquid caught in the tray. These grooves, machined in the surface of the insulating layer, will be filled at room temperature with pre-compressed insulating material, for example, glass-wool or rock-wool which will continue to fill out the grooves when the latter widen due to the shrinkage of the intermediary portions.

The insulating layer may also be provided with a reinforcing cloth or netting of high mechanical characteristics embedded in the mass of the insulating layer immediately below the bottom of the grooves to avoid the appearance of cracks below said bottom.

So that the leaking liquid caught in the tray does not spill due to the movements of the ship, the tray is provided with a brim around its periphery pointing upwards and/or towards the inside. The total collection of any leaks is assured by means of a deflecting plate which extends from the internal one of the shells forming the tank support and pours inside the free edge of the retaining brim of the tray.

The tray is also provided with a continuous exterior stiffening ring that may be situated on the free edge of the tray, preferably corresponding with the retaining brim or in an intermediate annular zone. This ring will normally function in compression. The purpose of this ring is to absorb membrane stresses that are produced in the tray due to its own weight, the weight of the collected liquid, and the vertical accelerations. Once installed in the ship, the tray will be freely supported on a discrete number of supports through the stiffening ring.

The stiffening ring will also have external projections running down or preferably outwards which will cooperate with lateral retention elements independent of the ring, to avoid the general displacements of the tray in any perpendicular direction to the axis of revolution of the tank, but allowing any movement of the tray parallel to the axis of revolution of the tank, as well as expansions and contractions of a radial nature.

An insulating material is provided between the stiffening ring and the supports as well as between the afore-mentioned projections and the retention elements. This material should have sufficient resistance in compression to bear the weight of the tray and collected liquid and also the accelerations parallel and perpendicular to the axis of revolution of the tank. This material may not exist if the insulating layer of the tray has been placed on the upper face.

The tray supports and the lateral retention elements may be joined to the internal shell of the tank support or to the pedestal structure. In either case the tray supports and the lateral retention elements may be either independent of each other or joined. If the supports are joined to the pedestal structure they will be joined to the brackets placed on the inside of the skirt or will go directly to the skirt if these brackets have not been installed or are of reduced dimensions.

Apart from the afore-mentioned supports hereinafter termed main supports, there are other secondary supports that can either be independent or combined with the main supports or lateral retention elements. These supports are used to install jacks which allow the tray to be taken up and lowered to test the upper face of the tray and the insulation of the tank, having first of all removed any insulating material placed among the main supports and the stiffening ring.

The object of the present invention, i.e., the ships construction with respect to the situation of the tanks and the supporting tray, will be more easily understood from the following description with reference to the enclosed drawings, which illustrate an embodiment given by way of non-limitative example.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view that partially shows the structure of the pedestal, the structure of the ship, and the lower part of the support.

FIG. 2 is a partial plan view of the ship.

FIG. 3 is a section on the line III-III of FIG. 2.

FIG. 4 is a similar view to FIG. 3 showing a variant embodiment.

FIG. 5 is a section on line V-V of FIG. 2.

FIG. 6 is a section on the line VIVI of FIG. 2.

FIG. 7 is a partial, schematic diametral section through the tank resting on its support with drip tray below.

FIG. 8 is a scaled-up detail of the supports of the drip-tray.

FIG. 9 is a view in the direction A of FIG. 8.

FIG. 10 is a section on the line X-X of FIG. 9.

FIG. 11 is similar view to FIG. 8 showing a variant embodiment.

FIG. 12 is a view similar to FIG. 1, but showing an embodiment of the invention in which portions of the double bottom and inner shell are omitted.

DETAILED DESCRIPTION OF THE DRAWINGS As it can be seen in FIG. 7, tank 1 is mounted on the ship 2 by means of a support made up of a peripheral structure 3, which forms a part of the tank shell 1. From this peripheral structure 3, two skirts or shells 4 and 5 go out. These skirts are stiffened at their lower part but not at the top. These skirts are used to transmit the corresponding forces to the ships structure.

In accordance with the invention, the pedestal for the support of shells 4 and 5 is made up as shown in FIG. 1 of a horizontal platform 6 and a continuous reinforced skirt 7 that runs between the platform 6 and the double bottom of the ship, the skirt being joined by its lower edge to the double bottom 8 and by its upper edge the platform 6. The upper edge of the skirt 7 is of the same perimeter as the lower edge of the internal shell 4 of the tank support and faces said lower edge of the shell 4. The platform 6 runs between the upper edge of the skirt 7 and the inner shell 9 of the hull.

Making reference to FIGS. 1 and 2, between platform 6 and the skirt 7, there are some flat plates 10 mounted perpendicularly to the base plane of the ship. These are represented by dotted lines. These plates are stiffened and face the floors and longitudinal girders. These plates 10 are welded by their inside edge of the skirt 7, by their upper edge to the platform 6, and by their lower edge to the top 8 of the double bottom. On their outside edge, the plates facing the floors are welded to the inner shell of the ship 9, and the outside edges of the places facing the longitudinal girders are welded either to the skirt of the adjacent corresponding tank or to the extreme transverse bulkheads in the cargo area.

The skirt or external shell 5 of the tank support may reach the platform 6 or is bottom edge may be spaced away therefrom. In the first case, when the skirt 5 extends as far as the platform 6, there is arranged beneath said platform a secondary small skirt 11 as shown in FIG. 3. This small skirt faces the afore-mentioned external shell 5 of the support. This secondary skirt has a height substantially smaller than that of the main skirt and is joined by its upper edge to platform 6. The secondary skirt 11 is also joined to the flat plates 10 at the lines of intersection with the same. This secondary skirt is joined to the main skirt using intermediate parts 12 to which are joined the vertical stiffeners 12' of the skirt. Preferably, as can be seen in FIG. 3, the internal edge of the platform 6 will be situated slightly inside the main skirt 7.

As shown in FIGS. 2 and 5 on the inside of the skirt 7 there are a series of brackets 13 having faceplates 14 and intermediate stiffeners 13'. for FIGS. 4 and 5 the stiffeners of the hull, outer shell, double bottom, etc. are not shown as these would be standard.

FIG. 5 which is a vertical section taken on the line VV of FIG. 2, shows the arrangement of these brackets 13, their faceplates l4 and the stiffeners 13'. In this figure there can be seen the arrangement of the plates 10 facing the longitudinal girders. Plates 10 are between two consecutive skirts 7 and 7 having a common platform 6.

In FIG. 6 which is a vertical section taken on the line VIVI of FIG. 2, the arrangement is also shown of the flat plates 10, in this case facing the floors, which by their external edges are welded to the inner shell 9 of the ship. The stiffening elements of the plates are shown as dotted lines 15, while the stiffening elements of the platform 6 are shown by numeral l5.

In FIG. 6 there can be seen the whole of the double bottom 8 and the, ships inner shell 9, in the area situated below the platform 6. However, the part of the double bottom and inner shell situated below the said platform can be suppressed in which case these parts are not reinforced, and then the floors, longitudinal girders and transverse webframes terminate in faceplates to which the corresponding edges of the flat plates 10 are welded. This means that if the double bottom is deleted the lower edges of the flat plates 10 are welded to the faceplate of the corresponding floor. If the inner shell 9 of the ship is omitted, the external edges of the flat plates 10 facing the floors are welded to the faceplates of the transverse web frames. See FIG. 12. There, it can be seen that the inner shell 9 is discontinuous below the horizontal platform 6, degrading into the faceplates 40 of the webframes 42. Also, except for an annular extension 44, the top of the double bottom 8 degrades into the faceplates 46 of the double bottom floors 48.

The skirt 7 may be cylindrical, as shown in FIG. 3, or take the form of an inverted cone as shown in FIG. 4 in order to match the shape of the bow and stem of the ship. If the outer one of the shells 5 that form part of the tank support does not reach the platform 6, the small skirt 11 shown in FIG. 3 is not used, but instead only the main skirt 7, complying with the afore-said requirements, with its stiffeners l2.

As can be seen in FIG. 7, there is a tray 17 below the tank 1 which is used to catch any leaks of transported liquid. This tray 17 is independent of the structure of the ship and of the tank support. To prevent the liquid caught in the tray from being split by the motions of the ship a peripheral rim 18 is added. This rim extends upwards and/or inwards. The collection of any leakage from the tray 17 is assured by means of a deflecting plate 19 that extends from the internal shell of the support into the inside of the free edge of the surrounding retention rim 18.

The tray 17 has an exterior continuous stiffening ring 20 that may be placed at the free edge of the tray as illustrated in FIG. 11, in which case it will correspond with the retaining rim 18 or be located in an intermediate annular zone as shown in FIGS. 7 and 8. The purpose of this ring which would normally work under compression is to absorb the membrane stresses which are produced in the tray by its own weight, the weight of the collected liquid, and the vertical accelerations.

The tray is mounted on supports 21 which can be joined to the brackets 13 or to the skirt 7 in the case where the structure of the pedestal is not fitted with such brackets or the brackets are of small dimension, or, alternatively, to the internal one of the two shells that comprise the support, this depending on whether the stiffening ring 20 is situated in an intermediate zone of the tray or at its free edge.

Referring to FIGS. 7, 8, 9 and 10, the main support 21 is made up of a continuous platform 22 with reinforcement 23 at its free edge and supporting elements 24 situated at intervals coniciding with the brackets 25 which are coplanar with the brackets 13.

Reinforcing ring 20 has a continuous band 26 and also a small continuous skirt 27 which are perpendicular to one another and reinforced with brackets 28 and 29. Extending from the ring 20 there are some projections 30 having an inverted profile section 31 between which and the platform 22 of the main support 21 there is an insulating material 32 of sufficient compressive strength to withstand the weight of the tray, the collected liquid, and the accelerations parallel and perpendicular to the axis of revolution of the tank. On both sides of each projection 30 there is a guide 33 which prevents lateral displacement of the tray but does allow axial displacement and radial expansion.

Apart from the main support already described, there are other secondary supports which, in this case, are constituted by the same platform 22 used for the main support with a bottom reinforcement 34. Between this secondary support and the rim 35 of the ring there are jacks 36 that allow the elevation or lowering of the tray after extraction of the filling material 32 for inspecting the upper face of the tray at any given moment. To locate the jacks 36 the secondary supports may be independent of the main supports 21. Also, the guides 23 can be independent of the supports 21. The stiffening ring 20 would then have projections housed between the guides 23 to prevent lateral displacements of the tray.

In the embodiment illustrated in FIG. 11, the supports take the form of a table or platform 22' joined to the internal shell 4 of the tank support which also has reinforcements 23'. The lateral retention is also obtained by means of guides of the same nature as those illustrated in FIG. 9. The jacks 36' are placed between the frame 37, shown by dotted lines, and the band 26'.

The main supports and the lateral retention elements or guides 23 can be joined to the internal shell of the tank support or to the pedestal structure. In both cases, the supports and the lateral retention elements can either be joined together or be independent. In the same way, the secondary supports can be independent or can be combined with the main supports or lateral retention elements.

The tray 17 will have an insulating layer 38 which, although in the described example is on the upper face of the tray, could equally well be located on the lower face. In the first case, the insulating layer will have a tight covering 39 able to withstand the low temperatures of any leaks from the tank. Consequently, the tray can be made from a normal steel or some other noncryogenic material. In the second case, if the insulating layer is on the underside of the tray, the tray should be made of a metallic material capable of resisting the low temperatures of the liquid and preferably be of the same material as the tank itself.

As already indicated, the tight covering 39 can be formed by one or two layers. In the first case, the tight covering is tightly connected to the drip tray along its whole periphery, so that the existence of leaks can be detected by introducing gas under pressure between the two elements. If the tight covering 39 is made up of two independent layers, these are joined together at their periphery so as to form a bag which, in its turn, will be joined to the tray along its periphery. This system makes it possible to determine, in case of a lask of the gas introduced between the tray and the adjacent tight covering, whether the leak is in the tray or in the lower layer of the tight covering. Shrinkage of the tight covering 39 will be absorbed by the portions of said tight covering which reproduce the shape of the knuckle between tray 17 and the surrounding retention rim 18.

The fact that the tray is independent of the tank allows said tank together with its support and corresponding tray to be assembled in a single operation. This is done either by lifting the assembly in which case the tray is attached to the support, at least during assembly operations, or by floating the assembly after having flooded the dock where the ship is being constructed. In the second case, the upper edge of the tray should be above the Waterline of the assembly and if the insulating layer is on the lower face of the tray it should be protected by a water-proof film.

So that the tray 17 can be constructed without stiffeners and made thin, without risk of buckling, during flotation of the assembly, at least the surface of the central part of the tray should be parallel to the bottom external surface of the tank so that the support area between tank and tray will be of sufficient magnitude. The rest of the tray can have a conical shape as shown in FIG. 7 tangential to the surface of the central zone. With the overall construction described the best transmission of forces through the support of the tank 1 to the ship is achieved retaining a simple construction of the ship structure.

As the tray is independent from the structure of the ship and outside the tank support, it can be built separately from the ship and installed jointly with the tank and the tank support. The support system for the tray is of a simple design thus assuring perfect functioning and allowing inspection of both upper and lower faces.

In the event that the lateral retention elements or guides 23 are independent of the main supports, the corresponding insulating material will be located so that in all conditions the structure of the ship will be protected against low temperatures.

I claim:

1. In ships having an independent pressurized cargo tank supported in a tank support comprising continuous shells, the improvement comprising a continuous skirt positioned on top of the double bottom floors of the ship and under said support shells, said skirt being stiffened and joined by its lower edge to said top of said double bottom and its upper edge being of the same perimeter as the lower edge of the lowermost internal shell in the tank support and disposed opposite it, a stiffened platform attached to said skirt and running between said upper edge and the inner shell of the hull,

a number of flat plates positioned between the platform and said skirt perpendicular to the base plane of said ship, said plates being stiffened and facing the floors and longitudinal girders and being welded by their internal and upper edges to said skirt and said platform, respectively, and by their bottom edge to the top of said double bottom, the external edges of said plates being welded to the inner shell of the ship, with the plates facing the floors, and to the skirt of an adjacent tank or to the end transverse bulkhead of the cargo tank zone, with the plates facing the longitudinal girders, a tray positioned under the tank independent of the structure of the ship and of the structure of the support, said tray having an insulating layer and a stiffening ring and being designed to catch possible small leaks of transported liquid, and a continuous leak deflection plate joined by its external edge to the inner shell of the tank support, the free edge of said tray being positioned next to the inner shell of the tank support below said deflec tion plate.

2. The combination of claim 1 in which the inside of the skirt has a series of brackets with faceplates and intermediate stifieners, the said brackets being coplanar with the aforementioned plates and being joined by their external edges to said skirt and by their lower edges to the top of said double bottom.

3. The combination of claim 1 in which a secondary skirt is positioned below said platform and facing the external shell of the support, said secondary skirt having a considerably lower height than the main skirt and being joined to said platform by its upper edge, said secondary skirt also being joined to said plates along the lines of intersection with the same and to said main skirt by means of intermediate parts.

4. The combination of claim 1 in which the internal edge of said platform is slightly inside the upper edge of said skirt.

5. The combination of claim 1 in which there is an intermediate deck between the 'outer and inner shells of the ship, facing the said platform.

6. The combination of claim 1 in which said skirt has a conical shape.

7. The combination of claim 1 in which said skirt has a cylindrical shape.

8. The combination of claim 1 in which the double bottom of said ship is omitted at least in the area located outside said skirt extending from the vicinity of the lower edge of said skirt, the lower edge of said plates being joined to the faceplates of the floors and longitudinal girders.

9. The combination of claim 1 in which the inner shell of said ship is omitted at least in the area located below said platform, the external edges of said plates facing said floors being joined to the faceplates of the transverse web frames.

10. The combination of claim 1 in which said tray has a surface which at least in its central zone is parallel to the bottom external surface of said tank so that when said tray is lifted it rests on the tank over said central zone, through the intermediate insulation, the peripheral zone of said tray having a conical shape tangential to the surface of the central zone.

11. The combination of claim 1 in which said tray is equipped, extending away from its free edge, with a continuous peripheral rim acting as a retaining element for liquid caught in the said tray and having a free edge at a distance from the external surface of said tank.

12. The combination of claim 1 in which said stiffening ring is located in the lower face of said tray and in the vicinity of the free edge of said tray.

13. The combination of claim 1 in which said stiffening ring is located on the lower face of said tray along an intermediate annular zone.

14. The combination of claim 1 in which said tray is supported freely on a number of main supports through said stiffening ring which has external projections which co-operate with lateral retention elements so arranged as to keep said tray in position and to allow displacements of the same parallel to the axis of revolution of said tank as well as radial expansion or contraction.

15. The combination of claim 14 in which an insulating material is arranged between said stiffening ring and said supports and also between said projections and said retention elements, said insulating material having sufficient compressive strength to withtand the weight of said tray, the collected liquid, and also the accelerations parallel and perpendicular to the axis of revolution of said tank.

16. The combination of claim 14 in which the supports which carry said tray and said lateral retention elements are joined to the internal shell of said tank support.

17. The combination of claim 14 in which the supports supporting said tray and said lateral retention elements are joined to said skirt.

18. The combination of claim 14 in which the supports supporting said tray and said lateral retention elements are joined to brackets on the inside of said skirt.

19. The combination of claim 14 in which there are secondary supports facing said stiffening ring between which supports and said ring there are positioned movable jacks which allow the raising and lowering of said tray.

20. The combination of claim 1 in which the insulating layer of said tray is on the upper face of the same and at least one independent tight layer is attached to said tray along its whole periphery and makes a hermetic seal delimited generally by said tray and above by said tight layer, there being at least one opening therein for the introduction of gas under pressure.

21. The combination of claim 1 in which the insulating layer of said tray is on the upper part of the same and there are disposed above the insulating layer two tight layers which are mutually independent, and also independent of said insulating layer, which are hermetically sealed to each other at their peripheries to form a bag with at least one opening for the introduction of gas under pressure, said bag in turn being joined at its periphery to said tray. 

1. In ships having an independent pressurized cargo tank supported in a tank support comprising continuous shells, the improvement comprising a continuous skirt positioned on top of the double bottom floors of the ship and under said support shells, said skirt being stiffened and joined by its lower edge to said top of said double bottom and its upper edge being of the same perimeter as the lower edge of the lowermost internal shell in the tank support and disposed opposite it, a stiffened platform attached to said skirt and running between said upper edge and the inner shell of the hull, a number of flat plates positioned between the platform and said skirt perpendicular to the base plane of said ship, said plates being stiffened and facing the floors and longitudinal girders and being welded by their internal and upper edges to said skirt and said platform, respectively, and by their bottom edge to the top of said double bottom, the external edges of said plates being welded to the inner shell of the ship, with the plates facing the floors, and to the skirt of an adjacent tank or to the end transverse bulkhead of the cargo tank zone, with the plates facing the longitudinal girders, a tray positioned under the tank independent of the structure of the ship and oF the structure of the support, said tray having an insulating layer and a stiffening ring and being designed to catch possible small leaks of transported liquid, and a continuous leak deflection plate joined by its external edge to the inner shell of the tank support, the free edge of said tray being positioned next to the inner shell of the tank support below said deflection plate.
 2. The combination of claim 1 in which the inside of the skirt has a series of brackets with faceplates and intermediate stiffeners, the said brackets being coplanar with the aforementioned plates and being joined by their external edges to said skirt and by their lower edges to the top of said double bottom.
 3. The combination of claim 1 in which a secondary skirt is positioned below said platform and facing the external shell of the support, said secondary skirt having a considerably lower height than the main skirt and being joined to said platform by its upper edge, said secondary skirt also being joined to said plates along the lines of intersection with the same and to said main skirt by means of intermediate parts.
 4. The combination of claim 1 in which the internal edge of said platform is slightly inside the upper edge of said skirt.
 5. The combination of claim 1 in which there is an intermediate deck between the outer and inner shells of the ship, facing the said platform.
 6. The combination of claim 1 in which said skirt has a conical shape.
 7. The combination of claim 1 in which said skirt has a cylindrical shape.
 8. The combination of claim 1 in which the double bottom of said ship is omitted at least in the area located outside said skirt extending from the vicinity of the lower edge of said skirt, the lower edge of said plates being joined to the faceplates of the floors and longitudinal girders.
 9. The combination of claim 1 in which the inner shell of said ship is omitted at least in the area located below said platform, the external edges of said plates facing said floors being joined to the faceplates of the transverse web frames.
 10. The combination of claim 1 in which said tray has a surface which at least in its central zone is parallel to the bottom external surface of said tank so that when said tray is lifted it rests on the tank over said central zone, through the intermediate insulation, the peripheral zone of said tray having a conical shape tangential to the surface of the central zone.
 11. The combination of claim 1 in which said tray is equipped, extending away from its free edge, with a continuous peripheral rim acting as a retaining element for liquid caught in the said tray and having a free edge at a distance from the external surface of said tank.
 12. The combination of claim 1 in which said stiffening ring is located in the lower face of said tray and in the vicinity of the free edge of said tray.
 13. The combination of claim 1 in which said stiffening ring is located on the lower face of said tray along an intermediate annular zone.
 14. The combination of claim 1 in which said tray is supported freely on a number of main supports through said stiffening ring which has external projections which co-operate with lateral retention elements so arranged as to keep said tray in position and to allow displacements of the same parallel to the axis of revolution of said tank as well as radial expansion or contraction.
 15. The combination of claim 14 in which an insulating material is arranged between said stiffening ring and said supports and also between said projections and said retention elements, said insulating material having sufficient compressive strength to withtand the weight of said tray, the collected liquid, and also the accelerations parallel and perpendicular to the axis of revolution of said tank.
 16. The combination of claim 14 in which the supports which carry said tray and said lateral retention elements are joined to the internal shell of said tank support.
 17. The combination of claim 14 in which thE supports supporting said tray and said lateral retention elements are joined to said skirt.
 18. The combination of claim 14 in which the supports supporting said tray and said lateral retention elements are joined to brackets on the inside of said skirt.
 19. The combination of claim 14 in which there are secondary supports facing said stiffening ring between which supports and said ring there are positioned movable jacks which allow the raising and lowering of said tray.
 20. The combination of claim 1 in which the insulating layer of said tray is on the upper face of the same and at least one independent tight layer is attached to said tray along its whole periphery and makes a hermetic seal delimited generally by said tray and above by said tight layer, there being at least one opening therein for the introduction of gas under pressure.
 21. The combination of claim 1 in which the insulating layer of said tray is on the upper part of the same and there are disposed above the insulating layer two tight layers which are mutually independent, and also independent of said insulating layer, which are hermetically sealed to each other at their peripheries to form a bag with at least one opening for the introduction of gas under pressure, said bag in turn being joined at its periphery to said tray. 